Dr. Tar Ortiz Peralta asked:
>We want to plot the function x=4 , but we can not do that. Do you know
>how to do it, help us please.
y -> x=4 is not a injective map. Thus, you must use "ParametricPlot":
ParametricPlot[ {4, y }, {y,-10,10} ]
See Mathematica help and The Book for more info.
Greetings,
Peter